The highest category in
the traditional Linnaean system of classification is the kingdom. At this level,
organisms are distinguished on the basis of cellular organization and methods of
nutrition. Whether they are single- or multiple-celled and whether they absorb,
ingest, or produce food are critical factors. Based on these types of distinctions,
the biological sciences define at least
five
kingdoms of living things:

Mostmacroscopiccreatures are either plants or animals. Of
course, humans are animals. The distinction between the plant and
animal kingdoms is based primarily on the sources of
nutrition and the capability of locomotion or movement.Plants produce new cell
matter
out of inorganic material by photosynthesis.
They do not have the ability to move around their environment except by growing or
being transported by wind, water, or other external forces.

Kingdom Animalia

Kingdom Plantae

In contrast,
animals do not produce their own food but must eat other organisms to obtain it.
Animals are generally more complex structurally. Unlike plants, they have nerves
and muscles that aid in rapid, controlled movement around their environment.
Animal cells usually do not have rigid walls like those of plants.
This accounts for the fact that your skin and flesh are flexible and the
trunk of a tree is not.

This simple dichotomy
between plants and animals is not adequate to encompass all life forms.
Some organisms have characteristics which do not qualify them to fit neatly
into either kingdom. For instance, funguses and most bacteria do not
photosynthesize and most of them lack a means of controlled locomotion.
Some organisms have
attributes of both plants and animals. For instance, there is a group
of common single-cell species living in fresh water ponds called Euglena
thatphotosynthesize and have their own means of locomotion
(whip-like tail structures called flagella). Because of these
and other exceptions, new kingdoms of living things had to be created.

Research done over the
last half century has shown us that there are even stranger
single-celled organisms known as archaeobacteria that live in extremely harsh
anaerobic environments such as hot
springs, deep ocean volcanic vents, sewage treatment plants, and swamp
sediments. Unlike other life forms, they usually
get their energy from geological sources rather than from the sun.
There are also microscopic things that are not quite alive by definition but
have some characteristics that are similar to living things.
These are the
viruses
and prions.
It is easy to overlook the importance of these extremely small things
because they cannot be seen with the naked eye. However, there are
very likely around ten times as many viruses as all living things put
together. There are about 50 million viruses in 1 cm³ of ocean
water. It has been estimated that these viruses are responsible for
the death of 20% of all oceanic bacteria every day, thereby keeping the
phenomenal reproductive capability of bacteria under control. There
are also complex interactions between bacteria, viruses, and other microbial
life forms within our own bodies. Most of the time, there are about 10
times as many microbial cells within us as there are body cells.

Phylum

Immediately below kingdom
is the phylum
level of classification. At
this level, animals are grouped together based on similarities in basic body plan or
organization. For instance, species in the phylum Arthropoda
have external
skeletons as well as jointed bodies and limbs.
Insects, spiders, centipedes, lobsters, and crabs are all arthropods.

Phylum Arthropoda

Phylum Mollusca

In contrast,
members of the phylum Mollusca have
soft, unsegmented bodies that are usually, but not always, enclosed in hard shells.
They also usually have at least one strong foot that helps them move.
Octopi, squids, cuttlefish, snails, slugs, clams, and other shellfish are
mollusks.

Bilateral symmetry(phylum Chordata)

There are at least 33 phyla (plural
of phylum) of animals. Humans are
members of the phylumChordata.
All of the chordates have elongated bilaterally symmetrical
bodies. That is to say, the left and right sides are
essentially mirror images of each other. If there are two functionally similar body
parts, they are usually found roughly equidistant from the center line, parallel to each
other. Note the location of the woman's eyes, nostrils, and cheeks relative to the
center line of her body.

Gill slits(phylum Chordata)

At some time
in their life cycle, chordates have a pair of lateralgill slits or pouches used to obtain oxygen in a liquid environment. In the case
of humans, other mammals, birds, and reptiles, lungs replace
rudimentary gill slits after the embryonic stage of development. Frogs
replace them with lungs in the transition from tadpoles to adults. Fish retain their
gill slits all of their lives.

Chordates also have a notochord
at some phase in their life cycle. This is a rudimentary
internal skeleton made of stiff cartilage that runs lengthwise under the dorsal surface of the body. Generally, there
is a single hollow nerve chord on top of the notochord. Among humans and the other vertebrates, the notochord is replaced by a more
complex skeleton following the embryonic stage of development.

Members of
the phylum Chordata also often have a head, a tail, and a digestive system with
an opening at both ends of the body. In other words,
the body organization is essentially that of a tube in which food enters one
end and waste matter passes out of the other. The chordates include mammals, birds,
reptiles, amphibians, fish, as well as the primitive lancelets(or amphioxus) and tunicates
(or sea squirts).

Notochord
in a lancelet(phylum Chordata)

Tunicate
(phylum Chordata)

Human
skeleton

Subphylum

The chordates are divided into three subphyla.
Humans are members of the subphylumVertebrata. Among the vertebrates,
the simple hollow dorsal nerve tube is replaced by a more complex
tubular bundle of nerves called a spinal cord.
A segmented vertebral
(or spinal) column of cartilage and/or bone develops around the spinal cord of
vertebrates to protect it from injury. At one end of the spinal cord is a head with
a brain and paired sense organs that function together to coordinate movement and
sensation.

Vertebrata is
the most advanced and numerous subphylum of chordates. It includes all of the fish,
amphibians, reptiles, birds, and mammals. Collectively, there are about 43,000
living vertebrate species in comparison to just over 1500 species in the other two invertebrate subphyla of chordates.

NOTE:Because science is constantly expanding our
knowledge of living things, the precise details of how organisms are
classified in the Linnaean system are frequently in flux. This is not due to confusion but rather to the
evolution of our understanding brought about by new discoveries.
For instance, as a result of the discovery of a
dramatically new form of life known as archaeobacteria, a growing number of researchers now
use a classification level above kingdoms referred to as a domain.
They define 3 domains of living things:Archaeo
(simple bacteria-like organisms that live in extremely harsh
anaerobic environments--these are the archaeobacteria),
Bacteria (all other bacteria, blue-green algae, and spirochetes), and
Eukarya (organisms with distinct nuclei in their cells--protozoans,
fungi, plants, and animals).